I only started training last month, so bear with me. I've been having similar problems with this technique and my sensei's suggestion was to practice the idea of cutting "through" uke. You want to make the movement as though uke isn't even there.

Using a bokken helped me to get the feel of the cutting through movement.

Since much of Aikido is an extension/derivation/distillation/whatever of Samurai sword work it's not surprising that it helps to get hold of a bokken and have a swing!